In general, a linear member as a base material is wound spirally to form a coil spring. In producing springs with non-linear elasticity characteristics, winding pitches and diameters of the linear member are changed according to the characteristics desired. However, this work is time-consuming and manufacturing costs will increase, so that product prices of the springs are pushed high.
On the other hand, when the spring is used as a tension spring and/or a torsion spring, its ends are bent in U-shape to form a hook, which is hooked or attached to an object. But, with this way of hooking, when a heavy load is applied to the hook, it might be unable to bear the load and be broken.
A flexible coupling, though not a spring, is discosed in Japanese Patent Publication No. 58-49732 which was laid open on Apr. 9, 1975, and announced to be granted on Nov. 7, 1983. It corresponds to U.S. Pat. No. 3,844,137. The coupling has the same operation as a spring in that both of them have elasticity. The coupling comprises a plurality of pairs of slits which are formed into a cylindrical main body. They have depths larger than the radius of the main body. They are arranged at the same pitch along the axis of the main body. Plate spring portions are defined respectively between one pair of the slits, respectively, and have the same width. Each pair has two slits extended in parallel with each other and from the opposite directions. The extending directions of each pair are shifted at the angle of 90 degrees compared with those of the adjacent pair.